Systems and Methods for Determining Stock Quantities Using a Capacitive Inventory Sensor

ABSTRACT

Systems and methods are provided for a capacitive inventory sensor. A system includes a track configured for retaining items. A first conducting plate is positioned along a portion of the track, and a second conducting plate is positioned in parallel with the first conducting plate along a portion of the track. The second conducting plate is positioned a distance from the first conducting plate, and the second plate is configured to have the items placed on top of the second plate. The system further includes a capacitance sensor configured for connection to the first and second conducting plates, where the capacitance sensor is configured to measure a capacitance between the first and second conducting plates, and where the capacitance varies based on a number of items positioned on the sensor track.

CROSS-REFERENCE TO RELATED APPLICATIONS

The application claims priority to U.S. Provisional Patent ApplicationNo. 61/493,190, filed Jun. 3, 2011, and entitled “Determining StockQuantities Using a Capacitive Inventory Sensor,” the entirety of whichis herein incorporated by reference.

TECHNICAL FIELD

The technology described herein relates generally to inventory detectionand more specifically to inventory detection based on capacitancevariation.

BACKGROUND

In retail environments, there are a variety of systems that mechanicallymove products so that they may be more easily seen and accessed bycustomers. These may be gravity fed, or have some sort of stored energy,such as a spring, which pushes product to the front of the display;hence these devices are frequently generically referred to as “pushers”.Though more expensive than simply depositing product on the bare shelvesand allowing customers to move items at their discretion, pushers arerapidly cost-justified for certain products by the resulting lift insales and reduced labor costs associated with restoring order to productthat has been “shopped.” Retailers are rapidly adopting pusher systemsand expanding their use within stores to include more product lines.

SUMMARY

In accordance with the teachings herein, computer-implemented systemsand methods are provided for a capacitive inventory sensor. A systemincludes a track configured for retaining items. A first conductingplate is positioned along a bottom portion of the track, and a secondconducting plate is positioned in parallel with the first conductingplate along the bottom portion of the track. The second conducting plateis positioned a distance from the first conducting plate, and the secondplate is configured to have the items placed on top of the second plate.The system further includes a capacitance sensor configured forconnection to the first and second conducting plates, where thecapacitance sensor is configured to measure a capacitance between thefirst and second conducting plates, and where the capacitance variesbased on a number of items positioned on top of the second plate.

As another example, a capacitive inventory sensor includes a trackconfigured for retaining items, where the track includes a pusher thatis configured to move along the track and to push the items toward thefront portion of the track as items are removed from the track. Astationary first conducting plate is positioned along a length of thetrack, and a stationary second conducting plate is also positioned alonga length of the track. A moveable third conducting plate is connected tothe pusher. The a face of the third conducting plate is positionedopposite a face of the first conducting plate and a face of the secondconducting plate such that the moveable third conducting plate overlapsa portion of the first conducting plate and a portion of the secondconducting plate. A capacitance sensor is configured to measure acombined capacitance formed among the first conducting plate, the secondconducting plate, and the third conducting plate, wherein the measuredcapacitance varies based on a position of the pusher along the track.

As a further example, a system for identifying a presence of an itemalong a transmission line includes a transmission line responsive to oneor more items positioned along the transmission line. An impulsegenerator is configured to transmit an impulse at a known frequency froma first end of the transmission line. A terminator is configured toreceive the impulse at a second end of the transmission line. An impulsedetector is positioned at the first end of the transmission line, andthe presence or absence of an item along the transmission line isdetermined based upon a signal detected by the impulse detector.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram depicting an environment that includes aninventory management system and capacitive inventory sensors.

FIG. 2 is a block diagram depicting an example environment for trackingproduct inventory using capacitive inventory sensors activated by aswitch at a transmitter.

FIG. 3 depicts a side view of an example configuration of a capacitiveinventory sensor.

FIG. 4 depicts a side view of an example configuration of a capacitiveinventory sensor that does not include a pusher.

FIG. 5 depicts an example capacitive inventory sensor.

FIG. 6 depicts an example capacitive inventory sensor with bottlesloaded onto the sensor.

FIG. 7 is a circuit diagram depicting an example configuration for acapacitive inventory sensor.

FIG. 8 is a graph depicting an example relationship between an A/Dconverter range and a first capacitive inventory sensor.

FIG. 9 is a circuit diagram depicting an example configuration for acapacitive inventory sensor that includes an add-on capacitor.

FIG. 10 is a graph depicting an example relationship between A/Dconverter values and a number of bottles of beverage present on acapacitive inventory sensor.

FIG. 11 is a graph depicting a relationship between an A/D converterrange and capacitance values of a capacitive inventory sensor that couldbe achieved using a differential sampling technique.

FIG. 12 is a graph depicting a sample advantage that could be realizedusing the differential sampling technique.

FIG. 13 depicts an alternative form for a capacitive inventory sensorthat utilizes a moveable capacitor plate.

FIG. 14 is a circuit diagram depicting an equivalent circuit to theconfiguration depicted in FIG. 13.

FIG. 15 is a graph depicting an example expected and measured variationin capacitance based on a position of the moveable plate.

FIG. 16 depicts an example three-plate capacitive inventory sensor.

FIG. 17 depicts a configuration where the stationary first plate andsecond plate extend the length of the capacitive inventory sensor andvary in width along that length.

FIG. 18 depicts a coded digital capacitive inventory sensorconfiguration.

FIG. 19 depicts an example implementation of a coded digital capacitiveinventory sensor.

FIG. 20 is a graph depicting measured data over the first four portionsin 0.25″ increments.

FIG. 21 is a block diagram depicting a transmission line reflectionsensing counter.

DETAILED DESCRIPTION

FIG. 1 is a block diagram depicting an environment that includes aninventory management system and capacitive inventory sensors. Retailersknow that much of the time some fraction of pushers or other inventoryholders are depleted or void of product and that this results in ashortfall in sales compared to what would have been possible with moreample stock. Some fraction of the empty pushers is due to the fact thatre-stocking personnel are unaware of the fact that the pushers areempty. Often, facilities upstream in the distribution system—and evenneighboring facilities in the same chain—have plenty of product but noawareness that a given store is void or nearly void. It is commonlyestimated that out-of-stocks (OOS) average around 10%, and fast-movinghigh-profile products suffer most. When a customer encountersout-of-stock conditions on a key item, not only do sales of that itemsuffer, many times the customer will shop elsewhere for that item, sosales of other items the customer would have bought in the originalstore are also lost. If OOS conditions occur too frequently, storeloyalty may erode.

In the absence of more timely, granular information about low-stockstatus, many retailers have minimized OOS probability by overstocking.This needlessly ties up valuable working capital, stocking space, shelfspace, and employee hours for over-inventory management. In cases whereproduct shelf life is short, overstocking results in expensive returnsor deep consumer discounts to incentivize quick sale. Extra facings of asingle product come at the expense of inventory variety that couldotherwise appeal to additional customers or potentially out-sell themarginal stock of the original brand.

Promotional or seasonal periods exacerbate the delicate balance betweenout-of-stocks and over-stocks. A successful promotion can double ortriple sales within a store. Some retailers resort to secondarylocations during a peak or promoted season. This can improve sales byallowing impulse purchases; however it can also depress sales ifretailers are unaware that the “home” location for a product is low orempty and transfer inventory from the secondary location to the placecustomers usually go to find the product. Because promoted and seasonalproducts make up a sizeable portion of retailer sales and profits, theconcept of “right-sized” inventory is elusive and transient.

Lost sales due to out-of-stocks are estimated in billions of dollarsannually. In fast-moving categories where consumers typically purchasemultiples of a given item, having only one item on the shelf is almostas consequential. When low-stock and over-stock situations are factoredin, the economic motivation to address replenishment deficiencies moreefficiently and immediately soars. A means to automatically alertresponsive personnel within and beyond the store has been unavailabledue to expense and lack of reliability. Many methods that have beenattempted also negatively impact the shopping experience or the ease ofrestocking. To be effective, stock-monitoring mechanisms can should beinexpensive, rugged, easy to retrofit to existing installations,unobtrusive, extensible to a variety of stock-monitoring conditions,reliable/accurate, and easy to integrate into the end users existing ITinfrastructure.

With reference to FIG. 1, an inventory management system 102 is incommunication with a receiver 104 and a data store 106 for managing andreceiving data about inventory distributed throughout an environment.For example, the inventory may be distributed throughout a factory, adistribution center, a store, or a single serving distribution machine.Based on the data received by the inventory management system 102 fromthe receiver 104 and the data store 106, the inventory management system102 may perform certain actions to maintain sufficient levels ofinventory throughout the environment. For example, when the inventorymanagement system 102 receives data that indicates that an inventorylevel at a particular location in the environment has run out or isrunning low on product, an alert can be issued that instructs andemployee to restock the particular location. The inventory managementsystem 102 may also receive data that indicates that the environment isrunning low on product and that more should be ordered. Such a decisionto order more product may be based on an amount of product detected, anordering threshold, an expected rate of product transfer/sales, adetected rate of product transfer/sales, etc.

The data store 106 for storing amounts of product detected and theinventory management system 102 for making inventory managementdecisions based on amounts of product detected are responsive to thereceiver 104, which is configured to wirelessly communicate with atransmitter 108. The transmitter is responsive to a plurality ofcapacitive inventory sensors (CIS) 110 that are configured to detect anamount of product present in an area based on a capacitance measurement.For example, a capacitor may be implemented such that the capacitancebetween its constituent plates varies based on an amount of weightplaced on top of the capacitor. In another configuration, one plate of acapacitor may be placed on a pusher with another one or more stationaryplates being positioned along the length of the track, where thecapacitance among the plates varies based on an overlap between thestationary plates and the pusher plate.

Each of the capacitive inventory sensors 110 includes a capacitancemeasurement device and a capacitor in some form. The capacitor has abase capacitance when an associated shelf or storage area is empty. Thatcapacitance changes when items are added or removed. In one example, thecapacitance changes in a linear fashion as items are added and removed.The capacitance measurement device may take the form of a capacitivesensor module (CSM), such as ones commonly utilized in MP3 players,smart phones, computer pads, computer displays, and lab test equipment.A capacitive sensor module detects when a finger, tap pen, or otheritems are touched to a section of a screen. A capacitive sensor modulecan be programmed to detect the change in capacitance above a thresholdlevel. When a threshold is exceeded, the module activates, sending asignal (e.g., a signal indicating the presence of absence of items). Inanother implementation, a micro-controller with an analog to digitalconverter (e.g., a 10-bit A/D converter), can be used to detect changesin capacitance with more granularity than the on-off detection providedby the capacitive sensor module.

The selection of the capacitor configuration to use for a particularproduct or location may be based on a number of factors. While pusherswork well with many products, there are also several instances where itis desirable to monitor the presence or absence of shelf stock whenthere are no pusher paddles present. It is common, for example, to findsingle-serve beverages or other items in a retail cooler that rely ongravity to pull them along a track or to the front of a shelf for easyconsumer access. In such cases, adding a pusher paddle may beprohibitively expensive or unreliable, as tracks often become stickyfrom product spills. Ease of restocking has also been found to beimpeded, in some cases, by the presence of pusher paddles. Lastly, thereare many areas of a retail environment that do not utilize pusherpaddles due to awkward product size or unmet sales volume criteria thatwould justify their use. It is also common for a given consumer productto be located in several places throughout a store. While one or more ofthose locations may have pusher paddles associated with them, it isoften the case that not all locations of a given product have pusherpaddles. Not only may it be important to note the stocking level of eachlocation, it may often be desirable to know the particular location thatis selling the most or least of a given item so allocated retail spacecan be adjusted to derive more sales within a given area or department.

FIG. 2 is a block diagram depicting an example environment for trackingproduct inventory using capacitive inventory sensors activated by aswitch at a transmitter. A plurality of capacitive inventory sensors 202are responsive to a transmitter 204 that selectively provides power tothe capacitive inventory sensors 202 from a battery 206 via a switch208. In response to a command (e.g., from the inventory managementsystem 210, in response to a processor at the transmitter 204, from aclock signal), a voltage measurement is taken to determine a capacitanceof one of the capacitive inventory sensors 202. The measured voltage,indicative of a capacitance at one of the capacitive inventory sensors202 and thus indicative of the amount of product present at thecapacitive inventory sensor 202 of interest, is transmitted via theantenna 214 of the transmitter 204 to the receiver 216, where such datais forwarded to the inventory management system 210 and the data store218. The capacitive inventory sensors 202 may be individually connectedto the antenna, or the sensors 202 may be connected to the antenna via aserial communication line, where sensors 202 that are currently notpowered via the switch 208 are in a high impedance state and will notinterfere with the currently powered sensor 202.

FIG. 3 depicts a side view of an example configuration of a capacitiveinventory sensor that includes a pusher. The capacitive inventory sensorincludes a capacitor that is formed from a top plate 302, a bottom plate304, and a dielectric layer 306, which could be formed from air oranother dielectric material. Units of product 308 are positioned on topof the top plate 302 and are forced toward a front portion of thecapacitive inventory sensor on a shelf 310 by a pusher 312. Acapacitance measuring device 314 is connected to the top plate 302 andthe bottom plate 304 to measure the capacitance formed by the top plate302, bottom plate 304, and dielectric 306. The capacitance varies basedon an amount of product 308 that is placed upon the top plate 302, ascertain characteristics of the capacitor 302, 304, 306 vary based uponweight being positioned on top of the top plate 302 (e.g., the distancebetween the top plate 302 and bottom plate 304 may decrease as moreweight is added). The capacitance varies in a predictable way, such thatan amount of product 308 positioned on top of the top plate 302 can bedetermined based on the magnitude of the capacitance change. Such datais forwarded by the capacitance measuring device 314 to the inventorymanagement system and data store for consideration and appropriateaction.

FIG. 4 depicts a side view of an example configuration of a capacitiveinventory sensor that does not include a pusher. The capacitiveinventory sensor includes a capacitor that is formed from a top plate402, a bottom plate 404, and a dielectric layer 406. Units of product408 are positioned on top of the top plate 402 and are forced toward afront portion of the capacitive inventory sensor by gravity based on theforward tilt of the top plate 402. A capacitance measuring device 414 isconnected to the top plate 402 and the bottom plate 404 to measure thecapacitance formed by the top plate 402, bottom plate 404, anddielectric 406. The capacitance varies based on an amount of product 408that is placed upon the top plate 402, as certain characteristics of thecapacitor 402, 404, 406 vary based upon weight being positioned on topof the top plate 402 (e.g., the distance between the top plate 402 andbottom plate 404 may decrease as more weight is added). Data isforwarded by the capacitance measuring device 410 to the inventorymanagement system and data store for consideration and appropriateaction.

FIG. 5 depicts an example capacitive inventory sensor. The examplecapacitive inventory sensor is configured for holding plastic or glassbottles. The capacitive inventory sensor includes a top plate 502 onwhich the bottles are placed. As bottles are placed on the top plate502, the capacitance formed by the top plate 502 and a bottom platepositioned under the top plate 502 changes. A capacitance measuringdevice may be connected at a back portion 504 of the capacitiveinventory sensor. The capacitive inventory sensor may be place on anincline, such as on a shelf in a convenience or grocery store cooler orwithin a single serving food/beverage machine (e.g., a pop machine) sothat bottles placed on the capacitive inventory sensor will tend toslide toward a front portion 506 of the capacitive inventory sensor.Molding 508 (e.g., plastic, metal, composite) may be incorporated withthe capacitive inventory sensor to hold the bottles within thecapacitive inventory sensor on the top plate 502.

FIG. 6 depicts an example capacitive inventory sensor with bottlesloaded onto the sensor. In the example of FIG. 6, a number of bottles602 are loaded into the capacitive inventory sensor on top of a topplate 604 that forms a capacitor with a bottom plate. A capacitancemeasuring device is connected at a back portion 606 of the capacitiveinventory sensor. Based on the capacitance measured, an inventorymanagement system may be able to sense whether any product is present onthe displayed capacitive inventory sensor and may further be able totell a number of units of product that are on the displayed capacitiveinventory sensor. Stocking and reordering operations may then becommanded accordingly.

FIG. 7 is a circuit diagram depicting an example configuration for acapacitive inventory sensor. The capacitive inventory sensor 702includes a capacitor 702 whose capacitance varies based on a condition.For example, the capacitance may vary as weight is added on top of oneof the plates of the capacitor 702 as described above. As a furtherexample, the capacitance may vary based on a position of one of theplates of the capacitor 702, as will be described in further detailherein below. The capacitance of the capacitor 702 is measured via amicrocontroller with a 10 bit A/D converter 704. To perform such ameasurement, the microcontroller 704 uses a switch 706 to charge asample and hold capacitor 708 to a voltage, V_(DD). The switch thentransitions to a second position, connecting the capacitive inventorysensor 702 in parallel with the sample and hold capacitor 708,distributing the charge between the sample and hold capacitor 708 andthe capacitive inventory sensor 702. The voltage then present at the A/Dconverter 710 is representative of the capacitance of the capacitiveinventory sensor 702, where a larger value of capacitance of thecapacitive inventory sensor 702 will result in a lower voltage sensed atthe A/D converter 710.

The voltage measured by the A/D converter 710 is related to thecapacitance of the capacitive inventory sensor 702 according to thefollowing formula:

V _(CIS)=(C _(r) *V _(DD))/(C _(CIS) +C _(r)),

where V_(CIS) is the voltage of the capacitive inventory sensor 702measured by the A/D converter 710, C_(r) is the capacitance of theinternal sample and hold capacitor and any add-on capacitors (noneincluded in FIG. 7), V_(DD) is the voltage used to charge the sample andhold capacitor 708, and C_(CIS) is the capacitance of the capacitiveinventory sensor 702.

FIG. 8 is a graph depicting an example relationship between an A/Dconverter's range and a first capacitive inventory sensor constructedsimilarly to the example of FIG. 7. In the present example, theeffective range of the capacitive inventory sensor is between 65 pF whenempty up to 145 pF when full. This range of capacitances is low on theA/D converter scale, which could result in difficulty in discerning thenumber of items present because the range of operation of the capacitiveinventory sensor uses such a small portion of the A/D converter range.It may be desirable shift the response curve to the right, such that amore dynamic range of A/D converter values is present in the 65 pF-145pF range.

Such shifting can be accomplished in a variety of ways, such as byincorporating an add-on capacitor in parallel with the sample and holdcapacitor. FIG. 9 is a circuit diagram depicting an exampleconfiguration for a capacitive inventory sensor that includes an add-oncapacitor. The capacitive inventory sensor 902 includes a capacitor 902whose capacitance varies based on a condition. The capacitance of thecapacitor 902 is measured via a microcontroller with a 10 bit A/Dconverter 904. To perform such a measurement, the microcontroller 904uses a switch 906 to charge a sample and hold capacitor 908 and anadd-on capacitor 910 to a voltage, V_(DD). The switch then transitionsto a second position, connecting the capacitive inventory sensor 902 inparallel with the sample and hold capacitor 908, distributing the chargebetween the sample and hold capacitor 908 and the capacitive inventorysensor 902. The voltage then present at the A/D converter 912 isrepresentative of the capacitance of the capacitive inventory sensor902, where a larger value of capacitance of the capacitive inventorysensor 902 will result in a lower voltage sensed at the A/D converter912. The presence of the add-on capacitor 910 shifts the relationshipshown in FIG. 8 to the right according to the relationship of theformula described above with respect to FIG. 7.

FIG. 10 is a graph depicting an example relationship between A/Dconverter values and a number of bottles of beverage present on acapacitive inventory sensor. The A/D converter 912 measures a voltagethat varies according to the capacitance of the capacitive inventorysensor 902. That capacitance varies in a predictable manner based on acondition, such as the number of bottles of beverage of a known weightpresent on one plate of the capacitive inventory sensor 902. Thatrelationship can be used to analytically or experimentally develop thegraph of FIG. 10, which maps voltages measured by the A/D converter 912to an amount of product present at the capacitive inventory sensor 902.Amounts of product or changes in the amounts of product can betransmitted to an inventory management system, which can analyze thevalues to provide data displays or make appropriate stocking/reorderingdecisions accordingly.

Other variations may also be made in a capacitive inventory sensorcircuit. In one example, the capacitive inventory sensor may be designedso achieve a linear response to a change in capacitance versus thechange in the number of items counted by the capacitive inventorysensor. The capacitive inventory sensor may be buffered from neighboringsensors so that a minimal reaction to items present at neighboringsensors is detected by the capacitive inventory sensor. It may furtherbe desirable to shield the capacitive inventory sensor from an expectedenvironment such as to compensate for metal shelving, radio frequencyinterference, etc.

FIG. 11 is a graph depicting a relationship between an A/D converterrange and capacitance values of a capacitive inventory sensor that couldbe achieved using a differential sampling technique. This approachrepeats the steps described above, where a filtered value is acquiredand stored for final calculations. This voltage sample is defined as V1.The process is then reversed, where the capacitive inventory capacitoris fully charged and the sample and hold or reference capacitor is fullydischarged. The reference capacitor is enabled and connected to thefully charged capacitive inventory sensor, resulting in another voltageto be sampled by the A/D converter. This voltage is defined as V2. Thefinal value is determined by computing the delta of the two samples(V2−V1). A graph of this technique is plotted in FIG. 11. FIG. 12 is agraph depicting a sample advantage that could be realized using thedifferential sampling technique. As shown in FIG. 12, the differentialsampling technique enables use of a broader portion of the A/D converterrange through the effective range of the capacitive inventory sensorcapacitance value, as indicated by the steeper slope of the differentialsampling technique plot. This steeper slope translates to an increase inresolution and accuracy of the capacitive inventory sensor.

In one example, the capacitive inventory sensor used for this data has asensor section comprising two plates of a capacitor running down amiddle portion of the sensor along the length of the sensor. The middleportion width dimension is 1.5″. Ground plates are positioned on eachside of the sensor separated by ⅛″ gap. A low cost micro-controller ispositioned on the back section of the capacitive inventory sensor. Othercomponents may include the external add-on capacitor and an interfaceconnector. Several of these capacitive inventory sensors can beconnected to a central collector which will control when each sensor ispowered up. Data can be collected and transmitted wirelessly (or in awired fashion) to a common node. The node will periodically transmit thedata to the inventory management system data servers for analysis andreport generation. A capacitive inventory sensor can be manufacturedusing a variety of materials, such as low cost plastic. The conductivegrounds and sensor section can be made by accurately spraying conductivepaint. Other materials could include flex circuit or even cardboard.

FIG. 13 depicts an alternative form for a capacitive inventory sensorthat utilizes a moveable capacitor plate. The capacitor inventory sensoris formed using three parallel plates. Two of the plates 1302, 1304 arefixed and stationary. The third plate 1306 is moveable, and may beconnected to a moveable member in a capacitive inventory sensor, such ason a pusher. As indicated in FIG. 13, as the moveable plate 1306 moves,the moveable plate 1306 overlaps differing portions 1308 of thestationary plates 1302, 1304. The differing amounts of overlap result indiffering levels of capacitance among the three plates 1302, 1304, 1306.Measurement of such capacitances can indicate the position of themoveable plate 1306, which can indicate the position of an object, suchas a pusher, to which the moveable plate 1306 is connected. The pusherposition can in turn be translated into an amount of product present inthe pusher assembly.

FIG. 14 is a circuit diagram depicting an equivalent circuit to theconfiguration depicted in FIG. 13. The two stationary plates can bearranged edge to edge so that the capacitance 1402 between the twostationary plates is small. The moveable plate is positioned oppositethe two stationary plates so that a face of the moveable plate isopposite the faces of the two stationary plates. Such a configurationforms two capacitors in series, as shown at 1404, 1406. A capacitancemeasuring device 1408 is configured to measure the capacitances 1402,1404, 1406 formed by the three plates.

The configuration shown in FIGS. 13 and 14 enables estimation of thecapacitance, C₂, between stationary plate 1 and moveable plate 3according to:

C ₂ =k*A ₁ /d,

where k is a constant, A₁ is the area of overlap between plates 1 and 3,and d is the distance of separation between stationary plates 1 and 2and moveable plate 3. Similarly, the capacitance between stationaryplate 2 and moveable plate 3 can be estimated as:

C ₃ =k*A ₂ /d,

where k is a constant, A₂ is the area of overlap between plates 2 and 3,and d is the distance of separation between stationary plates 1 and 2and moveable plate 3. A position for x=0 (where x is representative ofthe position of the moveable third plate, and the structure (e.g., thepusher) to which the third plate is attached) can be selected such that:

A=a*x,

where a is based upon by the widths of the plates and can be determinedusing a geometric calculation. Such selection results in:

C ₂ =C ₃ =k*a*x/d.

Simplifying k*a/d into a single constant m,

C ₂ =C ₃ =m*x.

Thus, the total capacitance, C_(m), seen by the measuring device 408 is:

C _(m) =C ₁ +m*x/2.

This is an equation of a straight line, where C_(m) is linearlyproportional to the displacement x with a known offset of C₁. Thisequation can be rearranged to identify the position, x as:

x=2(C _(m) −C ₁)/m.

Thus, the measured capacitance can be translated into a positionestimate.

FIG. 15 is a graph depicting an example expected and measured variationin capacitance based on a position of the moveable plate. The straightline 1502 is based on the above described linear equation. Thenon-straight but linearly trending line 1504 is based on actualmeasurements taken as a moveable third plate was translated with respectto a pair of stationary plates. The real-life measured capacitances 1504are sufficient for identifying an approximate position of the moveablethird plate. This position is related to the position of the structure,such as the pusher, to which the third plate is connected. Knowing thedimensions of product to be positioned in the capacitive inventorysensor, the position of the pusher can be translated to a number ofitems present in the capacitive inventory sensor.

FIG. 16 depicts an example three-plate capacitive inventory sensor. Twoof the plates 1602, 1604 are fixed and stationary. The third plate 1606is moveable, and may be connected to a moveable member in a capacitiveinventory sensor, such as on a pusher. As the moveable plate 1606 moves,the moveable plate 1606 overlaps differing portions of the stationaryplates 1602, 1604. The differing amounts of overlap result in differinglevels of capacitance among the three plates 1602, 1604, 1606.Measurement of such capacitances can indicate the position of themoveable plate 1606, which can indicate the position of an object, suchas a pusher, to which the moveable plate 1606 is connected. The pusherposition can in turn be translated into an amount of product present inthe pusher assembly.

The measured capacitance is based on the amount of overlap presentbetween the stationary first plate and second plate and the moveablethird plate. The amount of overlap among the plates can be varied inseveral different ways. As depicted in FIG. 13, the amount of overlapcan be varied where the moveable third plate extends beyond the lengthof the stationary first plate and the stationary second plate.

FIG. 17 depicts a configuration where the stationary first plate andsecond plate extend the length of the capacitive inventory sensor andvary in width along that length. A stationary first plate 1702 andstationary second plate 1704 are positioned along a length of thecapacitive inventory sensor track. The first and second plates 1702,1704 are largest at a back portion 1706 of the track and taper towardthe front portion 1708 of the track. The moveable third plate 1710 isconfigured to be attached to the pusher 1712, such that the third plate1710 traverses the length of the track with the pusher 1712. Thecapacitance between the stationary plates 1702, 1704 and the moveableplate 1710 increases as the third plate 1710 moves toward the backportion 1706 of the track because a larger area of first and secondplate conductors is directly opposite the face of the third plate 1710in the back portion position 1706. By measuring the capacitances amongthe three plates 1702, 1704, 1710 in a similar manner as described withrespect to FIG. 14, a position of the third plate 1710, and thus thepusher 1712 can be determined. The shape of the stationary first andsecond plates 1702, 1704 could be varied in other ways (e.g., in astepped fashion, in a curved fashion) to provide different levels ofdiscrimination, to provide greater resolution at a particular portion ofthe track, etc.

A position of the moveable third plate can also be determined using adigital coding configuration. FIG. 18 depicts a coded digital capacitiveinventory sensor configuration. A first stationary plate 1802 isprovided at a consistent thickness along the length of a capacitiveinventory sensor track. A plurality of additional stationary plates1804, 1806, 1808, 1810 have varying, stepped widths along the length ofthe track. A moveable third plate 1812 is configured to move along thelength of the track. When the moveable third plate 1812 is positionedopposite a wide portion of one of the additional stationary plates 1804,1806, 1808, 1810, a capacitance of known magnitude is formed between themoveable third plate and that stationary plate. When the moveable thirdplate 1812 is positioned opposite a narrow portion of one of theadditional stationary plates 1804, 1806, 1808, 1810, a capacitance ofsmaller magnitude is formed between the moveable third plate and thatstationary plate. A capacitance measuring device may be connected amongthe stationary first plate 1802, the moveable third plate 1812, and eachof the additional stationary plates 1804, 1806, 1808, 1810 in sequence,resulting in four capacitance measurements that, in combination, areindicative of the position of the moveable third plate 1812. Based onthe on-off indication associated with each of the additional stationaryplates 1804, 1806, 1808, 1810, the position of the moveable third plate1812 can be decoded. In the example of FIG. 18, the wide and narrowportions of the additional stationary plates 1804, 1806, 1808, 1810, arearranged in a Gray code, where only one additional stationary plate1804, 1806, 1808, 1810 has a bit (portion) that changes in each positionalong the track. In some configurations, the bits (portions) may beunequally spaced to provide higher resolution at different portions ofthe track.

FIG. 19 depicts an example implementation of a coded digital capacitiveinventory sensor. The example includes a rectangular piece ofsingle-sided copper-laminated board from which copper has beenselectively removed to leave a reference track and four code tracks. Thecode areas are one inch long. The moveable capacitor plate is made froma rectangle of the same material, with the copper covered with Kaptontape to form a dielectric. FIG. 20 is a graph depicting measured dataover the first four portions in 0.25″ increments. The changes from “0”to “1” in the code can be seen.

FIG. 21 is a block diagram depicting a transmission line reflectionsensing counter. When a short (in time) electrical impulse is applied tothe terminals of a transmission line using a pulse generator 2102, thepulse will travel along a transmission line 2104 at a speed equal to thespeed of light multiplied by the “velocity factor” of the transmissionline 2104. If the line 2104 is of uniform characteristic impedance, thisimpulse looks the same at each point along the transmission line 2104,apart from attenuation due to the characteristics of the materials thatmake up the transmission line 2104, and a time delay. If the impulseencounters a portion of the line 2104 where there is an impedancedifferent from the so-called “characteristic impedance of the line”, theimpulse splits into two components. One continues on down the line 2104,slightly diminished in amplitude (called the forward pulse), and theother is reflected back toward the input end of the transmission line(the reflected pulse). If the input end of the cable is observed, saywith an oscilloscope or impulse detector 2106, the source pulse and thereflected pulse may be observed, separated in time. The time differencebetween them is equal to twice the travel time from the input end of thetransmission line 2104 to the source of the reflection. This approachhas been used for decades to locate the source of problems ontransmission lines 2104, cables etc. Under appropriate conditions, atransmission line 2104 having multiple impedance discontinuities willshow reflections from all of these. A transmission line 2104 which isinfinitely long or one that is terminated with an impedance 2108 equalto its characteristic impedance will have no reflections. The approachworks well in all types of transmission lines, such as coaxial cable,parallel wire transmission line, microstrip line, and twisted pairs.Coaxial cable confines the fields associated with transmission ofsignals entirely within its structure. Some types of transmission line,such as parallel wire line and microstrip line have fields which extendsignificant distances beyond the physical extents of the transmissionline. Objects 2110, such as units of product on a shelf 2112, nearby theline 2104 will interact with the field and cause the impedance tochange, generating a source for a potential reflection of a pulse.

This gives rise to the notion of using such a line 2104 to monitor thepresence or absence of objects 2110 in its vicinity. For instance, asuitable transmission line 2104 could be embedded in the shelf 2112 of adisplay on which products 2110 are to be placed, and fitted at one endwith a pulse generator 2102 and a means of detecting reflections 2106and at the other end with a termination 2108. With no stock on the shelf2112, the pulses are all absorbed by the termination 2108. As product2110 is placed on the shelf, reflections will begin to appear. Theimplementation issues in doing this are mostly to do with therequirement for very precise resolution in time. Using the rule of thumbthat in one nanosecond an electromagnetic wave travels a foot in freespace, one can see that if the objects 2110 to be monitored are inchesin diameter, resolution of better than one nanosecond is required. Thisis no problem for lab equipment, particularly if a human isinterpreting, say, an oscilloscope display. Trying to do thisautomatically and inexpensively using limited electrical power is not aneasy task. Apart from the pulse generator 2102, there is the need for aso-called directional coupler connected between the pulse generator 2102and the input of the transmission line 2104, and a high-speedhigh-resolution analog-to-digital convertor (ADC) 2106. After the pulseis generated, the ADC 2106 samples the reflected signal arm of thedirectional coupler until all reflections have died away. Successivesamples are stored in computer memory, and can then be processed by acomputer program to reveal the impedance discontinuities that reveal thepresence of product 2110 on the shelf 2112.

In one example, the signal at the reflected arm of the directionalcoupler can be passed to a simple diode detector. All the reflectedsignals can be rectified and averaged. Under conditions of no stock, theoutput from the detector 2106 was low. As stock was added, the signallevel rose. Such detection may work best as a “none or some” monitor. Ifthe output of coupler is digitized with sufficient resolution,identification of the location of individual items can be achieved.

Another use of impulse functions is in determining the frequencyresponse of networks. This can be accomplished by using mathematicalproperties of the Fourier Transform. Applying the Fourier Transform(actually the so-called Fast Fourier Transform, or FFT) to atime-sequence of samples yields a frequency-sequence of results. Thetime response and frequency response form a so-called Fourier Transformpair. In general, the more compressed a signal is in time, the morespread out it is in frequency, and vice versa. An infinitely narrowimpulse has a uniform spectrum from DC to infinite frequency. A narrowpulse in the nanosecond range will have a spectrum stretching out beyond1 GHz. Hence a network, such as a filter, can be subjected to a singleimpulse, and then its frequency response is simply the FFT of the outputof the network. In this way, the frequency response can be obtained veryconveniently, without having to use a swept source to measure at everyfrequency.

The present approach is to do this the other way round. First thetransmission line is swept, recording the output of the directionalcouplers reflected arm over a wide range of frequencies. The amplitudeand phase of the steady state response is recorded. The results are, infact, the same coefficients that would have been determined using theimpulse method. By applying an FFT to the steady state coefficientsthen, one obtains the impulse response of the transmission line—asequence of values stretching over a time interval. This set of valueswill look just like the TDR results, but obtained in a quite differentway. If a low power, wide frequency range network analyzer is built, itcan be used to obtain the required frequency response values. Withrecent advances in Direct Digital Synthesis (DDS) integrated circuits,building a simple vector network analyzer that uses modest amounts ofpower is becoming easier and less expensive.

TDR is a promising way to not only count products on a self, but alsodetermine where on the shelf they are. As discussed here, it can be doneeither in the traditional way using an impulse or by using a vectornetwork analyzer and applying an FFT to the results.

It should be understood that as used in the description herein andthroughout the claims that follow, the meaning of “a,” “an,” and “the”includes plural reference unless the context clearly dictates otherwise.Also, as used in the description herein and throughout the claims thatfollow, the meaning of “in” includes “in” and “on” unless the contextclearly dictates otherwise. Finally, as used in the description hereinand throughout the claims that follow, the meanings of “and” and “or”include both the conjunctive and disjunctive and may be usedinterchangeably unless the context expressly dictates otherwise; thephrase “exclusive or” may be used to indicate situation where only thedisjunctive meaning may apply.

1. A capacitive inventory sensor, comprising: a track configured forretaining items; a first conducting plate positioned along a bottomportion of the track; a second conducting plate positioned in parallelwith the first conducting plate along the bottom portion of the track,wherein the second conducting plate is positioned a distance from thefirst conducting plate, and wherein the second plate is configured tohave the items placed on top of the second plate; and a capacitancesensor configured for connection to the first and second conductingplates, wherein the capacitance sensor is configured to measure acapacitance between the first and second conducting plates, wherein thecapacitance varies based on a number of items positioned on top of thesecond plate.
 2. The sensor of claim 1, further comprising atransmitter, wherein the transmitter is configured to transmit an alertindicative of a number of items retained by the track.
 3. The sensor ofclaim 2, further comprising a data processor configured to determine anumber of items retained by the track based on the measured capacitance.4. The sensor of claim 3, wherein the processor is configured to causethe transmitter to transmit the alert when the number of items meets oneor more threshold criteria.
 5. The sensor of claim 3, wherein theprocessor is configured to cause the transmitter to transmit the alertwhen the number of items is equal to zero.
 6. The sensor of claim 1,wherein a face of the first place is positioned opposite a face of thesecond face plate.
 7. The sensor of claim 1, further comprising a singleserving beverage dispensing machine.
 8. The sensor of claim 1, whereinthe track includes a front portion, and wherein the track is configuredto travel the items toward the front portion of the track as items areremoved from the track.
 9. A capacitive inventory sensor, comprising: atrack configured for retaining items, wherein the track includes a frontportion, and wherein the track includes a pusher that is configured tomove along the track and to push the items toward the front portion ofthe track as items are removed from the track; a stationary firstconducting plate positioned along a length of the track; a stationarysecond conducting plate positioned along a length of the track; amoveable third conducting plate connected to the pusher, wherein the aface of the third conducting plate is positioned opposite a face of thefirst conducting plate and a face of the second conducting plate suchthat the moveable third conducting plate overlaps a portion of the firstconducting plate and a portion of the second conducting plate; acapacitance sensor configured to measure a combined capacitance formedamong the first conducting plate, the second conducting plate, and thethird conducting plate, wherein the measured capacitance varies based ona position of the pusher along the track.
 10. The sensor of claim 9,further comprising a data processor configured to determine a number ofitems present on the track based on the measured capacitance.
 11. Thesensor of claim 10, further comprising a data processor configured todetermine the position of the pusher based on the measured capacitance.12. The sensor of claim 11, wherein the position of the pusher isdetermined according to:x=2(C _(m) −C ₁)/m, where x is the position of the pusher, C_(m) is themeasured capacitance, C₁ is the capacitance between the first conductingplate and the second conducting plate, and m is a constant.
 13. Thesensor of claim 12, whereinm=k*a/d, where k is a constant, a is an area of a face of the firstconducting plate overlapped by the third conducting plate.
 14. Thesensor of claim 9, further comprising a force mechanism, wherein theforce mechanism is configured to force the pusher toward the frontportion of the track as items are removed from the track.
 15. The sensorof claim 14, wherein the force mechanism comprises a spring.
 16. Thesensor of claim 9, wherein the face of the first conducting plate andface of the second conducting plate are substantially rectangular. 17.The sensor of claim 9, wherein the face of the first conducting platevaries in width along the length of the track.
 18. The sensor of claim9, wherein the second conducting plate is discontinuous along the lengthof the track.
 19. The sensor of claim 18, further comprising: astationary fourth conducting plate positioned along the length of thetrack, wherein the third conducting plate is discontinuous along thelength of the track, wherein the capacitance sensor further measures asecond combined capacitance formed among the first conducting plate, thethird conducting plate, and the fourth conducting plate, wherein theposition of the pusher is determined based upon the measured capacitanceand the measured second capacitance.
 20. A system for identifying apresence of an item along a transmission line, comprising: atransmission line responsive to one or more items positioned along thetransmission line; an impulse generator configured to transmit animpulse at a known frequency from a first end of the transmission line;a terminator configured to receive the impulse at a second end of thetransmission line; an impulse detector positioned at the first end ofthe transmission line, wherein the presence or absence of an item alongthe transmission line is determined based upon a signal detected by theimpulse detector.
 21. The system of claim 20, wherein the transmissionline is embedded in a shelf of a product display.